Normal cell division needs the formation of a mitotic spindle apparatus, which is directly related to microtubule (MT) assembly. Thus, if the MT assembly is inhibited by tubulin-binding agents, the mitotic spindle functions will be perturbed, resulting in the inhibition of cell division at the metaphase/anaphase transition of mitosis. Therefore, tubulin-binding agents are anti-cancer drugs that can inhibit cell division by perturbing the MT assembly and cause cancer cell death. However, the current tubulin-binding anti-cancer drugs cannot specifically recognize cancer cells and serve as imaging probes. Our lab has recently used phage display technique to successfully identify two types of peptides: tubulin-binding peptides and SKBR-3 breast cancer cell-targeting/internalizing peptides. Moreover, we have developed expertise in the synthesis of lanthanide ion doped upconversion nanoparticles (UCNPs) and their utilization as cancer cell imaging probes. In contrast to the down-conversion nanoparticles such as the widely used quantum dots (QDs), the UCNPs can be excited by a longer wavelength light such as near infrared (NIR) light (e.g., 980 nm) to emit a shorter wavelength light (e.g., green light). Due to their ability to be excited by NIR light, whic can penetrate cells and tissues relatively deeply, UCNPs can be used for cell/tissue imaging without causing either autofluorescence from or photodamage to the biomolecules, cells and tissues. This project is based on these successes and will conjugate both tubulin-binding and cancer-targeting/internalizing peptides onto the surface of UCNPs. Our hypothesis is that core-shell upconversion nanoparticles (Rare-earth doped ?-NaYF4:Yb,Er upconversion nanocrystal as a core and silica as a shell) with both tubulin-binding and SKBR-3 breast cancer cell- targeting/internalizing peptides conjugated to the surface will (1) recognize the breast cancer cells and be internalized; (2) bind tubulins to interrupt intracellular MT assembly, inhibit cell proliferation and cause cell death in vitro and in vivo; and (3) enable the selective fluorescent imaging of the cancer cells and tumor tissues under NIR excitation. We will carry out two specific aims: (1) Aim 1: Evaluate the in vitro MT assembly in the presence of UCNPs with both tubulin-binding peptides and SKBR-3 cancer cell-targeting/internalizing peptides conjugated to the surface to understand how cell-targeting tubulin-binding UCNPs interrupt MT assembly in vitro. (2) Aim 2: Evaluate in vitro SKBR-3 breast cancer cell proliferation and cell cycle as well as in vitro and in vivo targeted cancer cell imaging and killing after SKBR-3 cells interact with the cell-targeting tubulin-binding UCNPs in vitro or in vivo. This project will advance the targete cancer treatment and diagnosis by developing targeted cancer imaging and therapeutic agents. The cancer-targeting tubulin-binding UCNPs developed in this project are multi-functional theranostic agents that can target and kill cancer cells and at the same time be fluorescently detected and tracked inside the cancer cells and tumor tissues.